High frequency transformer core comprised of magnetic fluid



OF MAGNETIC FLUID Filed May 25, 1964 11 R T AM Y! %w :u U Em G. R. MOHRHIGH FREQUENCY TRANSFORMER CORE COMPRISED DEVICE Nov. 22, 1966CIRCULATING INVENTOR Glenn R. Mohr WWW Jim )9" ATTORNEY United StatesPatent 3,287,677 HIGH FREQUENCY TRANSFORMER CORE COM- PRISED OF MAGNETICFLUID Glenn R. Mohr, Linthicum, Md, assignor to Westinghouse ElectricCorporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed May25, 1964, Ser. No. 369,979 2 Claims. (Cl. 336-57) This invention relatesto electric induction devices, and in particular to electrical inductorsand transformers.

Presently, high kva., high frequency current transformers for inductionheating employ low permeability air cores. Under matched conditionsthese air cores yield a coupling coeflicient of .7 to .8, and thus asecondary to primary kva. ratio of 20% to 25%. This ratio is muchsmaller under unmatched conditions. Magnetic materials having a muchhigher permeability than air could be employed to improve the secondaryto primary kva. ratio. However, magnetic cores used in this applicationgenerally are subject to the following limitations.

The inductance of such a device is a direct function of the product ofthe core cross section and the core permeability. Because of the highfrequencies involved in this application, a small inductance isrequired. The cross-section, permeability product must be kept low. Fora core composed of a high permeability material the requiredcross-section becomes so small that insulation between windings becomesdiflicult.

The inductance is also a direct function of the number of conductivewindings. For this application involving high frequencies and a highpermeability core, the low inductance required means fewer windings. Ifthe number of conductive windings is allowed to approach unity, theturns ratio also approaches unity. Current transformers must have alarge primary to secondary turns ratio.

Hysteresis losses, generated during inductive coaction between theconductive windings and the core material, cause hot spots in the corematerial. These hot spots are especially damaging at the highfrequencies involved in the application.

It is, therefore, an object of this invention to provide an improvedhigh frequency transformer or inductor.

Another object of this invention is to provide a core material havingmagnetic and fluid properties.

Another object of this invention is to provide a magnetic fluid to beemployed as the core material in high frequency transformers andinductors and to have a permeability lower than that of core materialscurrently used by higher than the permeability of air.

Another object of this invention is to provide a magnetic fluid to beemployed as the core material in high frequency transformers andinductors and which functions in a heat removing relationship within thetransformer or inductor.

A further object of this invention is to provide a high frequencytransformer which has a higher coupling coefficient and a correspondinghigher secondary to primary kva. ratio than heretofore possible.

A further object of this invention is to provide a high frequencytransformer or inductor of a smaller size than heretofore possible.

Further objects, features and advantages of this invention will becomeapparent to those skilled in the art upon further study of thespecification.

Briefly, in accordance with this embodiment of the invention, theseobjects are attained by'circulating a magnetic fluid within a conduitmeans comprised of, a circulating or flow forcing means, a heatgenerating core Patented Nov. 22, 1966 zone, and a heat exchanger means.The magnetic fluid is continuously displaced from the core zone into theheat exchanger means by the circulating means. The magnetic fluid isheated while flowing through the core zone and releases this heat as itflows through the heat exchanger means. The particular value of thepermeability of the magnetic fluid is suflicient to increase thecoupling coefficient over that of air, but not high enough to require acore cross-section as small as that of high permeability materialspreviously used.

For a clearer understanding of the invention, reference may be made tothe following detailed description and accompanying drawing.

The drawing illustrates magnetic fluid 1 contained within a conduit 2. Acirculating device 3 forces the flow of magnetic fluid 1 throughentrance port 4 into core zone 5 including flow path 6, flow path 7,which form closed magnetic path 8, out exit port 9, and into flow path10 through a heat exchanger 11. Two conductive windings 12 are shown inthis embodiment.

The magnetic fluid 1 is composed of small particles of a magneticmaterial suspended in an appropriate fluid. The magnetic materialsupplies the permeability necessary for induction action occurring incore zone 5. The fluid portion of the magnetic fluid 1 suspends andtransports the magnetic material throughout the conduit means 2, andprovides electrical insulation and mechanical lubrication between theparticles of magnetic material.

The flow paths 6 and 7 form a path for the closed magnetic path 8. Thismagnetic path has a low reluctance due to the magnetic fluid 1 containedtherein which expedites the establishment of the magnetic fields of theelectric current in conductive windings 12. The conductive windings 12must be disposed for inductive coaction with magnetic path 8 andaccordingly may be wound around flow path 6 or 7, or both.

Heat is generated in the portion of magnetic fluid 1 which comprises theclosed magnetic path 8. The sources of this heat are eddy currents andhysteresis losses incurrent during the inductive action. Copper lossesin the conductive windings caused by the current therein may contributea small amount of heat. At the power levels and frequencies involved inthis application this heat is excessive and will damage the core zone 5if not removed therefrom.

The excess heat is removed by circulating magnetic fluid 1 around acooling loop described below. The heated magnetic fluid comp-rising themagnetic path 8 is dis-placed from the core zone 5 in the flowingaction. This heated magnetic fluid flows through the heat exchangerwhich extracts the excess heat. The cooler magnetic fluid from the heatexchanger flows through the circulating device 3 which supplies theforce causing the flow. The cooler magnetic fluid from the circulatingdevice 3 displaces hot magnetic fluid from the core zone 5 thuscompleting the cooling loop around the conduit 2.

The use of magnetic cores in high frequency inductive devices results incertain limitations as stated previously. The insulation limitation dueto the small size of the core cross section is overcome by employing themagnetic fluid 1. The permeability of the magnetic fluid 1 is less thanthe permeability of previously employed magnetic cores. For the sameinductance, the magnetic fluid 1 allows a larger core cross-section. Theunity turns limitation is also overcome by employing the magneticfluid 1. The lower permeability of the magnetic fluid 1 allows moreconductive windings for the same inductance. A lower operatingtemperature is obtained by circulating the magnetic fluid 1 andcontinuously displacing hot magnetic fluid from the core zone.

The correct functioning of this invention requires that the magneticfield 1 possess magnetic and fluid properties. Any singular, or anycombination of substances having these properties will suflice. Acomposition of 80% finely powdered ferrite and 20% oil, has a relativepermeability of 10 to 20 and is useful in many high frequencyapplications. It is to be understood, however, that this invention isnot to be so limited to the combination of ferrite and oil. Othersubstances could be employed and include, any finely powdered materialhaving magnetic and fluid characteristics, and any powdered magneticmaterial suspended in a gas or dispersed in a liquid.

That it also be understood that the particular conduit 2 discussed isnot critical to the operation of this invention and the invention is notto be so limited. More flow paths through the core zone forming aplurality of closed magnetic paths may be employed. A plurality ofcooling loops, each containing a circulating device and a heat exchangermay be employed. The conductive windings 12 may be inside the conduit 2,or external, or both; and must be at least one in number.

The conduit 2 may be of the form of any volume containing a circulatingdevice. The conductive windings would be enclosed within the volume, andwould be geometrically positioned with one another so as to have acommon magnetic path. Heat exchange would occur on the exterior surfacesof the volume.

Another form of the conduit to be included in this invention is a singleclosed loop, similar to a doughnut. The closed loop would provide boththe closed path for the closed magnetic field and the cooling loop.Conductive windings, circulating means, and heat exchanger means wouldbe included along the closed loop.

The correct functioning of this invention requires relative motionbetween the conductive windings 12 and the magnetic fluid 1. In theembodiment discussed the conductive windings 12 were stationary and themagnetic fluid 1 moved in relation thereto. An arrangement wherein theconductive windings 12 move and the magnetic fluid 1 is stationary is tobe included in this invention.

This invention is not peculiar to transformers and inductors, butincludes any induction device having operating limiting characteristicssuch as described.

Although this invention has been described with re- 4 spect toparticular embodiments thereof, it is not to be so limited as changesand modifications may be made therein which are within the full intendedscope of the invention as defined by the dependent claims,

I claim as my invention: 1. In a high frequency induction device inwhich heat is gene-rated due to the induction action therein, thecombination comprising:

a conduit means formed by a first section and a second section and athird section all connected in parallel;

magnetic fluid contained within said conduit means, the magnetic fluidin said first and second sections forming a relatively low reluctanceclosed magnetic path;

at least one conductive winding wound in an inductive relation about atleast one of said sections included in said magnetic path, said at leastone winding adapted to be energized by a high frequency current, theinduction action between the current in said at least one Winding andsaid magnetic fiuid generating heat in the magnetic fluid forming saidmagnetic path;

flow forcing means connected in said third section for circulating saidmagnetic fluid around a heat removing flow path formed by said thirdsection in series with the parallel combination of said first and saidsecond section to continuously remove and replace the heated magneticfluid by forcing said magnetic fluid to flow from said third sectionthrough said first and second sections in the same direction and thenreturn to said third section;

heat exchange means connected in said third sec' tion for removing theheat from the heated magnetic fluid therein as said magnetic fluidcirculates around said heat removing flow path.

2. The combination as specified in claim 1 wherein two conductivewindings are wound in an inductive relation about at least one of saidsections included in said magnetic path.

References Cited by the Examiner UNITED STATES PATENTS 2,440,556 4/1948Paluev 33657 2,607,542 8/1952 Spellman l9221.5 2,651,258 9/1953 Piercel92-2l.5 X

FOREIGN PATENTS 394,858 12/1908 France.

636,127 4/1960 Great Britain.

LEWIS H. MYERS, Primary Examiner.

T, J. KOZMA, Assistant Examiner.

1. IN A HIGH FREQUENCY INDUCTION DEVICE IN WHICH HEAT IS GENERATED DUETO THE INDUCTION ACTION THEREIN, THE COMBINATION COMPRISING: A CONDUITMEANS FORMED BY A FIRST SECTION AND A SECOND SECTION AND A THIRD SECTIONALL CONNECTED IN PARALLEL; MAGNETIC FLUID CONTAINED WITHIN SAID CONDUITMEANS, THE MAGNETIC FLUID IN SAID FIRST AND SECOND SECTIONS FORMING ARELATIVELY LOW RELUCTANCE CLOSED MAGNETIC PATH; AT LEAST ONE CONDUCTIVEWINDING WOUND IN AN INDUCTIVE RELATION ABOUT AT LEAST ONE OF SAIDSECTIONS INCLUDED IN SAID MAGNETIC PATH, SAID AT LEAST ONE WINDINGADAPTED TO BE ENERGIZED BY A HIGH FREQUENCY CURRENT, THE INDUCTIONACTION BETWEEN THE CURRENT IN SAID AT LEAST ONE WINDING AND SAIDMAGNETIC FLUID GENERATING HEAT IN THE MAGNETIC FLUID FORMING SAIDMAGNETIC PATH; FLOW FORCING MEANS CONNECTED IN SAID THIRD SECTION FORCIRCULATING SAID MAGNETIC FLUID AROUND A HEAT REMOVING FLOW PATH FORMEDBY SAID THRID SECTION IN SERIES WITH THE PARALLEL COMBINATION OF SAIDFIRST AND SAID SECOND SECTION TO CONTINUOUSLY REMOVE AND REPLACE THEHEATED MAGNETIC FLUID BY FORCING SAID MAGNETIC FLUID TO FLOW FROM SAIDTHIRD SECTION THROUGH SAID FIRST AND SECOND SECTIONS IN THE SAMEDIRECTION AND THEN RETURN TO SAID THIRD SECTION; HEAT EXCHANGE MEANSCONNECTED IN SAID THIRD SECTION FOR REMOVING THE HEAT FROM THE HEATEDMAGNETIC FLUID THEREIN AS SAID MAGNETIC FLUID CIRCULATES AROUND SAIDHEAT REMOVING FLOW PATH.